This application claims priority to Japanese Patent Application No. 11-103239 filed on Apr. 9, 1999, which is incorporated herein by reference.
The present invention relates to a film for stretch-wrapping. More particularly, the invention relates to a film for stretch-wrapping comprising chiefly an ethylenexe2x80x94(meth)acrylic acidxe2x80x94(meth)acrylic acid ester terpolymer or a metal salt ionomer thereof.
PVC films have heretofore been chiefly used as stretchable films (for wrapping foamed polystyrene trays containing foods) for business use such as in supermarkets and convenience stores. However, from the environmental viewpoints in recent years, it has been urged to provide a substitute for the PVC. In the field of stretchable films for stretch-wrapping, the PVC films have rapidly been substituted by the olefin resin films.
Many films for stretch-wrapping have so far been proposed using an ethylene/unsaturated carboxylic acid copolymer, an ethylene/unsaturated carboxylic acid/unsaturated carboxylic acid ester terpolymer or an ionomer thereof.
Japanese Unexamined Patent Publication (Kokai) No. 134591/1978 discloses a film for wrapping of an ethylene copolymer comprising (a) an ethylene, (b) an unsaturated carboxylic acid alkyl ester, (c) an unsaturated carboxylic acid and (d) a metal salt of an unsaturated carboxylic acid, the component (a) being contained in an amount of from 90 to 98 mol %, the component (b) being contained in an amount of from 9.7 to 2.0 mol %, the component (c) being contained in an amount of from 0 to 2.5 mol %, and the component (d) being contained in an amount of from 0.3 to 2.5 mol %.
Japanese Laid-Open Patent Publication for PCT application (Kohyo) No. 506820/1992 discloses a film for wrapping comprising:
(a) at least 80% of a terpolymer of at least 50% by weight of an ethylene, 2 to 20% by weight of an unsaturated carboxylic acid having 3 to 8 carbon atoms, and 2 to 20% by weight of at least one kind of an alkyl acrylate or an alkyl methacrylate in which the alkyl group has 2 to 12 carbon atoms, or a moiety derived from a mixture thereof, 0 to 10% of the acid group of the acid group-containing moiety thereof being neutralized with at least one kind of metal ions; and
(b) 0.1 to 2% by weight of at least one kind of a sorbitan fatty acid ester;
at least one surface of said film being treated with corona to a degree sufficient for producing a wet tensile force of 40 to 50 dynes/cm on the treated surface thereof.
Japanese Examined Patent Publication (Kokoku) No. 100741/1995 discloses a film for stretch-wrapping obtained by inflation-molding an ethylene methacrylate copolymer resin containing 8 to 12% by weight of a methacrylic acid and having a tensile stress of 1.1 to 1.5 times in both the machine and transverse directions when stretched by 100% and having a tensile stress of 1.5 to 2.0 times in the transverse direction when stretched by 200% with respect to the stress of when it is stretched by 50%.
Japanese Patent No. 2642583 discloses a film for stretch-wrapping comprising chiefly 50 to 99% by weight of an ethylenexe2x80x94(meth)acrylic acid copolymer resin and 50 to 1% by weight of an ethylenexe2x80x94(meth)acrylic acidxe2x80x94(meth)acrylic acid ester terpolymer resin and, optionally, containing an anti-fogging agent (a defogging agent) or a tackifier. There have been described that the above bipolymer alone or the terpolymer alone exhibits a 100% tensile stress that does not lie within a suitable range (110 to 170 kg/cm2 in the machine direction of the film, and 60 to 110 kg/cm2 in the transverse direction), and is not suited for the automatic wrapping.
The ethylenexe2x80x94(meth)acrylic acid bipolymer generally has a high stiffness, whereas the ethylenexe2x80x94(meth)acrylic acidxe2x80x94(meth)acrylic acid ester terpolymer has a low stiffness and is soft. This holds even for the ionomers obtained by neutralizing the above copolymers with metal ions. The ionomer of the above bipolymer is called hard ionomer, and the ionomer of the above terpolymer is called soft ionomer.
The film for stretch-wrapping disclosed in the specification of Japanese Patent No. 2642583 is obtained by mixing the above bipolymer and the terpolymer at a predetermined ratio so as to be suited for the automatic wrapping. However, these two copolymers are not necessarily compatible to each other and, hence, the external haze value tends to increase. Therefore, the film is not still satisfactory from the standpoint of seeing through the wrapped content.
Further, the film for stretch-wrapping is usually blended with an anti-fogging agent to improve the property for seeing through the content. When the ionomers of the above-mentioned copolymers are used, however, gel tends to be formed during the molding operation with the passage of time, making it difficult to prepare the film itself.
It is therefore an object of the present invention to provide a film constituted by the above-mentioned terpolymer or an ionomer thereof, having a large tensile stress in the machine direction, excellent mechanical properties and stretch-wrapping property, i.e., to provide a film for stretch-wrapping that is less broken at the time of wrapping, that can be favorably torn off in the transverse direction, that can be intimately adhered on the container walls to be wrapped, and that excellently restores its shape after it is depressed and deformed as a result of wrapping.
Another object of the present invention is to provide a film for stretch-wrapping that can be easily formed and easily treated, that is homogeneous even optically, exhibiting less external haze and enabling the content to be seen through.
A further object of the present invention is to provide a film for stretch-wrapping, which suppresses the gelling tendency when it is blended with an anti-fogging agent without impairing the appearance or without being foamed during the working.
According to the present invention, there is provided a film for stretch-wrapping formed of a resin composition containing, as a chief component, an ethylene/(meth)acrylic acid/(meth)acrylic acid ester terpolymer that contains not more than 7% by weight and, particularly, less than 5% by weight of a (meth)acrylic acid ester unit, and, optionally, containing an anti-fogging agent or a tackifier.
It is desired that the terpolymer used in the present invention contains from 5 to 20% by weight of a (meth)acrylic acid, and not less than 0.1% by weight but less than 5% by weight of a (meth)acrylic acid ester.
The film for stretch-wrapping can be prepared by the inflation method. However, the terpolymer used in the invention can be molded into a film by the T-die method offering an advantage that the film can be produced at a high speed.
According to the present invention, there is further provided a film for stretch-wrapping formed of a resin composition containing, as a chief component, an ionomer obtained by ionizing an ethylene/(meth)acrylic acid/(meth)acrylic acid ester terpolymer that contains less than 5% by weight of a (meth)acrylic acid ester unit, and, optionally, containing an anti-fogging agent or a tackifier.
It is desired that the ionomer used in the invention uses, as a base, a copolymer that contains from 5 to 20% by weight of a (meth)acrylic acid and not less than 0.1% by weight but less than 5% by weight of a (meth)acrylic acid ester, and has an ionization degree of from 0.1 to 30%.
This ionomer, too, has an advantage that it can be molded into a film by the T-die method.